Belphegor is a very special case in terms of how he is fought: he will absorb all damage done to him, except for his weakness. On top of that, his weakness will change whenever he is successfully damaged. His weakness will cycle between Fire, Water, Lightning, and back to Fire, so in order to defeat him, he needs to be attacked with these elements while following the cycling of his weaknesses. Aside from that, Belphegor fights like most of the other Nemeses.
He uses Hammer to inflict physical damage on one character, Diffusion Ray for damaging the whole party, and the "sin"-based ability Sloth, which dispels all characters' buffs and hinders them by applying a weakness to every element. He also has Firaja, Blizzaja, and Thundaja, so bring a Vampire (or someone with the Absorb Genome ability) in order to obtain them. Lastly, he can summon three spheres that share the same damage/weakness-cycling gimmick that will fight alongside him. The spheres attack with the same -ja spells that Belphegor has and will also Self-Destruct after some time.
The first thing to do is bring a Freelancer and use Examine, then Flee. Though this is not really necessary as his weakness only changes whenever he takes damage, not when he absorbs damage. Focus on magic-users is a top priority. The best team to use consists of Vampires with access to the Swordmaster's Bushido commands and a Spiritmaster for using Stillness with the Hasten World perk which is always handy.
When the battle begins, set up the Vampires to use Free Lunch, Firaja, Blizzaja, then Thundaja. Regardless of the weakness Belphegor starts with, this will set him up to successfully damage him, and the spheres he summons. An alternative to using Vampires, if one does not have access to the -ja spells yet, is to use Black Mages while utilizing Firaga, Blizzaga, and Thundaga instead of the former spells. With these, however, one have the choice to either target Belphegor directly or damage everyone at once. The spheres will self-destruct in due time so, if one is focusing on just Belphegor, they pretty much take care of themselves provided Stillness is still up.
A downgraded version of Belphegor can also be encountered with severely decreased stats. In addition, the only -ja spell that Belphegor [-] possesses is Firaja, severely hindering its usefulness in obtatining Genome Abilities.
[view·edit·purge]In demonology, Belphegor is a demon and one of the seven princes of Hell who suggests ingenious inventions that will make people rich, although Bishop and witch-hunter Peter Binsfeld believed Belphegor to tempt by means of laziness. According to Peter Binsfeld's Binsfeld's Classification of Demons, Belphegor is the chief demon of the deadly sin Sloth.
Belphegor originated as the Assyrian Baal-Peor, the Moabitish god to whom the Israelites became attached in Shittim (Numbers 25:3), which was associated with licentiousness and orgies. As a demon, he is described in Kabbalistic writings as the "disputer," an enemy of the sixth Sephiroth "beauty." When summoned, he can grant riches, the power of discovery and ingenious invention. His role as a demon was to sow discord among men and seduce them to evil through the apportionment of wealth.